1. Field of the Invention
The present invention relates to a method for deriving information on exercise and physical activity induced changes in body fatigue, wherein parameters from the measurement of a physiological signal are obtained as input and these parameters being information on the intensity of exercise or physical activity. More generally speaking the invention relates to the monitoring of body functions, in particular to methods that are aimed to describe exercise and physical activity. The invention relates also to a method for deriving predictions on body fatigue and recovery during physical exercise and while recovering from such exercise. The term “body fatigue” means here also its counterpart, homeostatic disturbance induced by physical activity.
The current invention presents a procedure for predicting body fatigue during exercise and recovery from exercise on the basis of physiological measurement.
2. Description of the Prior Art
The control of exercise intensity, duration, and information from fatigue and recovery from the exercise are key elements in maintaining and achieving a good physical fitness and especially important in health enhancing physical activity, wherein exercise is directed to improve health and fitness. In particular, for individuals that suffer from some clinical condition, such as cardio-vascular disease, it is extremely important to maintain physical activity within safe limits. In athletic sports, disturbance of homeostasis induced by training exercises is also important for attaining a training effect.
The accumulation and reduction of the effects of exercise on the body is described in this document by two related concepts, body fatigue and recovery. Body fatigue is defined as a decrease of physiological resources due to the effects of exercise and physical activity. Recovery from physical exercise is defined as the restoration of physiological resources that has been used during the accumulation of body fatigue during exercise.
It has been generally accepted that especially the balance between exercise and recovery is important in athletic training and sports. Optimal training requires a disturbance of homeostasis and enough rest to recover from the exercise. This may be a hard goal to achieve for a person engaged in physical exercise and training, since exercise methods are mainly based on experience and general knowledge on the physiology. If exercise bouts are too mild, no positive training effect occurs because disturbance to the homeostasis has been minimal. On the other hand, if exercise bouts are scheduled too severe and too frequent, negative training effect may occur because bodily functions have not been restored properly. To gain positive training effect exercise bouts must be scheduled optimally, in order to give the body a chance to adapt a new level of functioning.
To summarize, a method that would give feedback on body fatigue and time required for recovery on the basis of individual's own physiological characteristics and responses to exercise would be certainly helpful to many individuals engaged in health enhancing physical exercise and fitness training and would potentiate more optimal and safe training schedules.
Oxygen consumption (VO2), that is, the rate of oxygen intake, is a central mechanism in exercise and provides a measure to describe the intensity of the exercise. Oxygen is needed in the body to oxidize the nutrition substrates to energy and therefore VO2 is very tightly coupled with the energy consumption requirements triggered by exercise and physical activity. American College of Sports Medicine Position Stand recommendations for exercise prescription (ACSM 1998) suggests the use of VO2 for the measurement of physical activity.
The level of oxygen consumption can be measured by different methods. The most accurate methods rely on the measurement of heat production or analysis of respiratory gases but require heavy measuring equipment and are therefore restricted to the laboratory environment. There are also more cost effective and practical means to estimate oxygen consumption using indirect methods based on the measurement of, for example, heart rate, ventilation, skin temperature, or movement. In particular, there is a close relationship between heart rate and oxygen consumption during exercise as increased oxygen consumption in the muscles requires an increase in circulatory volume. Heart rate is a major determinant of the circulatory volume and often provides a reasonable estimate of the oxygen consumption.
Maximal oxygen consumption (VO2max) is defined as the maximal rate of oxygen intake during exhaustive exercise and denotes person's ultimate capacity for aerobic energy production. Usually this is achieved by stepwise exercise protocol to a voluntary exhaustion (maximal exercise stress test), during which the oxygen uptake is measured. Also non-exercise methods are available to estimate person's VO2max based on individual characteristics such as, for example, age, sex, anthropometric information, history of physical activity, or resting level physiological measurements (e.g. Jackson et al. 1990).
Knowing the absolute oxygen consumption rate at which a person is exercising and the maximal attainable oxygen consumption of the same person, exercise intensity can be described as a percentage of the maximum This is crucial, as maximal values of VO2 can vary markedly between subjects. Thus, two persons that differ in their maximal VO2 but exercise at the same relative intensity have similar exercise impact on their bodies.
Athletic training and physical exercise in general has acute effects on body resources and body fatigue. The accumulation of body fatigue is depends on and determined by the characteristics of the exercise, including intensity, duration, and phase of the exercise. At high exercise intensities the energy requirements increase and induce a proportional reduction of available body resources. The mobilization of body resources is associated with accelerated physiological function and involves increased levels of oxygen consumption, circulation, ventilation, and hormone secretion (e.g., catecholamines). Metabolic function during exercise is characterized by increased rate of energy release from carbohydrates and body fats, and involve also by-products such as lactate, all of which reduce the level of metabolic resources available in the body.
The physiological processes of recovery from exercise involve a renewal of consumed body resources and are generally characterized as opposite to those during exercise. The level of physiological function shows attenuation towards normal levels. The recovery of metabolic resources involves replenishment of energy stores (e.g., glycogen) and removal of exercise-induced by-products (e.g., lactate). The process of recovery requires oxygen and therefore VO2 and heart rate remain elevated after exercise and may be used as composite indicators of the replenishment of the resources in the body. This indicates that the extend of exercise induced fatigue may be determined by the characteristics of the recovery process after the exercise.
The prior art has also documented some work on the measurement of exercise levels and stress on the basis of heart rate variability (HRV). HRV denotes the extent of rhythmic changes evident in the heart rate. The relationship of the heart rate variability to the exercise and stress is well known and documented in the prior art. Golosarsky and Wood (U.S. Pat. No. 5,891,044), Heikkilä and Pietilä (U.S. Pat. No. 6,104,947), and Hoover (U.S. Pat. No. 6,212,427) have all implemented a technique of determining the stress caused by exercise using different types of indices based on HRV. These methods usually require preset individual thresholds and state declarations, as defined by the user or history values, to give an estimate of the level of stress caused by the exercise and workload. The described methods are relatively simple, easy to implement and provide feedback on the acute exercise load.
It has been shown earlier that the amplitude of the HRV is associated with the intensity of physical activity. It is also known that HRV is associated with the aerobic threshold of the metabolism, which usually occurs at approximately 50–75% of maximal intensity in exercise (Tulppo et al., 1996). It is therefore clear to anyone experienced in the art that the HRV is primarily a measure of the intensity of the exercise and therefore provides little if any information on the dynamic phenomena of accumulation of body fatigue during different phases of the exercise. Thus, the described measures are primarily dependent on the instantaneous characteristics of the exercise and are not capable of adapting to temporal dynamics in different phases of the exercise. For example, during a short but intensive exercise HRV reflects high stress than considerably longer exercise with lower intensity, although in this case the longer duration exercise could accumulate, in effect, higher levels of body fatigue and a longer time required for recovery.
Prior art has documented work on deriving information on the accumulation of body fatigue and exhaustion as due to physical workload. Bernard, Sherwin, Kenney, William and Lewis (U.S. Pat. No. 4,883,063) have presented a method for monitoring heat stress, as especially occurring in a hot factory environment. The levels of heart rate and skin temperature are used within predefined temporal window to monitor potential exhaustion and a warning is triggered if a predefined threshold value is passed. The solution also includes an assessment of recovery on the basis of heart rate measurement, during which the person has to stay at rest for few minutes.
It is apparent to one skilled in the art that the method of Bernard et al. is designed for the analysis of tonic workload with known properties (e.g., heat stress). In most real life occasions, intensity of the exercise may vary markedly with different phases of the exercise due to, for example, conditions (e.g., up- and down hills), training mode (walking and running), or any means of controlling exercise intensity due to, for example, sports characteristics, physiological state or training protocol. The method of Bernard et al. is dependent on the instantaneous levels of the heart rate and skin temperature and therefore, in a similar manner to the methods based on HRV, does not include history information on the accumulation of body fatigue. The method may provide reliable results within constant working environment with known workload, but it is clearly not sufficient for monitoring body fatigue during exercise, wherein the level of heart rate is heavily dependent on the intensity of the exercise and thus does not indicate level of exhaustion.
The method presented by Bernard et al. has also some limitations with regards to the monitoring of recovery. The estimation of the recovery is somewhat problematic in the described method, since it requires few minutes of rest and is not therefore applicable to continuous monitoring of recovery within dynamic changes in exercise phases and intensities. In general, the method does not involve a differential estimation of the recovery component, which impairs the estimation of the recovery during dynamic exercise, wherein a decrease in exercise intensity may induce a reduction in recovery state. All this implies that the described method is not capable of producing continuous information on recovery and does not predict the amount of recovery required in advance to the onset of actual recovery.
To summarize, the monitoring of exercise effects on the body is not possible with a model that does not take into account the fact that exercise has a cumulative impact in the accumulation of the body fatigue and that it is not equal at different intensities and phases of the exercise. The description of the prior art clearly indicates that the described methods are highly dependent on the exercise state and do not contain cumulative information on the accumulation of fatigue through the exercise. The described methods neither do potentiate a continuous monitoring of recovery, which would be most important in any condition wherein the exercise is dynamic and the user would benefit from the information on the onset and progress of recovery.